To establish and maintain a robust link between a wireless communication receiver and a wireless communication transmitter in a wireless communication network, the reference frequency from the local oscillator at the receiver must attain high levels of precision and accuracy. Frequency drift in both idle and active modes can result in dropped calls and missed pages. In the past, Voltage Controlled, Temperature Compensated Crystal Oscillators (VCTCXOs) have been employed in wireless communication receivers to provide a high precision frequency reference to the receiver. Recently, manufacturers have begun to replace relatively expensive VCTCXOs with less expensive Digitally Controlled Crystal Oscillators (DCXOs) at the cost of less precision. Therefore, to prevent frequency drift, most wireless communication receivers use some type of automatic frequency correction circuit to correct frequency errors in the local oscillator.
During start-up, and initial cell selection, the Automatic Frequency Correction (AFC) circuit is used to tune the local oscillator to the cell frequency. Once the local oscillator is synchronized to the correct frequency, the wireless communication device tracks the frequency error by periodically estimating the frequency offset. To enable frequency offset estimation, the base station sends known training symbols to the wireless communication device. By correlating received training symbols with the expected training symbols, an estimate of the frequency offset can be obtained. The frequency offset is input to the AFC circuit to correct for any frequency errors.
In GSM networks, the radio frequency is divided into a series of frames. Each frame comprises eight time slots. A Global Systems for Mobile communication (GSM) data block comprises 456 bits and require four time slots to transmit. The data transmitted in a single time slot is referred to as a data burst. The normal GSM data burst includes 26 training symbols occupying the approximate center of each data burst. Thus, a data block typically includes 104 training symbols that are used for frequency offset estimation.
In most scenarios, the 104 training symbols in a data block are sufficient to enable frequency offset estimation. However, there are some circumstances where reception of a data block may be terminated early in order to conserve battery power. For example, reception of an empty paging block (EPB) transmitted on the Paging Channel (PCH) may be terminated after reception of the first data burst. Also, the wireless communication device may terminate reception of a coded data block after reception of two or three data bursts upon successfully decoding of the data block. In these and similar scenarios where reception of a data block is terminated early, the number of training bits available for frequency offset estimation is reduced and the accuracy of the frequency offset estimation is degraded. The degradation can be severe in idle mode, especially when the wireless communication device is in Discontinuous Reception (DRX) mode. In DRX mode, the wireless communication device is in sleep mode for a large percentage of the time and receives data blocks very infrequently.